📚 KS3 CIE Balancing Equations: Step-by-Step Guide | KS3 CIE 化学方程式配平：分步指南
Balancing chemical equations is one of the most important skills in KS3 CIE Science. It shows that atoms are rearranged during a chemical reaction, not created or destroyed. This guide explains the rules, worked examples and common pitfalls so you can balance equations confidently.
配平化学方程式是 KS3 CIE 科学中最重要的技能之一。它表明原子在化学反应中被重新排列，而不是被创造或消灭。本指南讲解规则、例题和常见易错点，帮助你自信地配平方程式。
1. What Is a Chemical Equation? | 什么是化学方程式？
tances formed are called products. An arrow (→) separates the reactants from the products.
化学方程式用符号和化学式来描述化学反应。起始物质称为反应物，生成的新物质称为生成物。箭头（→）将反应物与生成物分开。
For example, the reaction between hydrogen and oxygen to make water can be written in words as:
例如，氢气与氧气反应生成水的文字表达式为：
hydrogen + oxygen → water
→ H₂O before balancing. The numbers in the formulae are subscripts and cannot be changed.
用符号表示，在配平前为 H₂ + O₂ → H₂O。化学式中的数字是下标，不能随意更改。
In a balanced equation, coefficients are placed in front of formulae to show the correct ratio of particles. For KS3 CIE, a balanced equation has the same number of each type of atom on both sides.
在配平的方程式中，系数放在化学式前面，以表示粒子的正确比例。在 KS3 CIE 中，配平的方程式两边每种原子的数目相同。
2. Word Equations First | 先写文字表达式
Before balancing, write a word equation using the correct chemical names. This helps you identify the reactants and products clearly. For KS3 CIE, you will often be given word equations or asked to interpret them.
在配平之前，先用正确的化学名称写出文字表达式。这有助于你清楚地识别反应物和生成物。在 KS3 CIE 中，你经常会遇到给出文字表达式或要求解释文字表达式的题目。
Example: magnesium reacts with oxygen to form magnesium oxide.
例子：镁与氧气反应生成氧化镁。
magnesium + oxygen → magnesium oxide
Then replace each name with its correct formula: Mg + O₂ → MgO.
然后将每个名称替换为其正确的化学式：Mg + O₂ → MgO。
Always check that you have written the correct formula before you start changing coefficients. A wrong formula cannot be fixed by balancing.
在开始改变系数之前，务必检查你是否写出了正确的化学式。错误的化学式无法通过配平来修正。
3. Symbols and State Symbols | 化学式与状态符号
Each element is represented by a symbol. Compounds use formulae made from element symbols and subscript numbers, such as H₂O, CO₂ and NaCl. You must know the common formulae before balancing.
每种元素都有一个符号。化合物使用由元素符号和下标数字组成的化学式，例如 H₂O、CO₂ 和 NaCl。在配平之前，你必须熟悉常见的化学式。
State symbols are often added in brackets after each formula: (s) for solid, (l) for liquid, (g) for gas and (aq) for aqueous solution. They do not affect balancing.
状态符号通常写在化学式后的括号中：(s) 表示固体，(l) 表示液体，(g) 表示气体，(aq) 表示水溶液。它们不影响配平。
Some common formulae you will meet in KS3 CIE are listed below:
以下是 KS3 CIE 中你会遇到的一些常见化学式：
| Substance | 物质 | Formula | 化学式 | State at room temperature | 室温下状态 |
|---|---|---|
| Water | 水 | H₂O | liquid | 液体 |
| Carbon dioxide | 二氧化碳 | CO₂ | gas | 气体 |
| Methane | 甲烷 | CH₄ | gas | 气体 |
| Oxygen | 氧气 | O₂ | gas | 气体 |
| Hydrogen | 氢气 | H₂ | gas | 气体 |
| Nitrogen | 氮气 | N₂ | gas | 气体 |
| Hydrochloric acid | 盐酸 | HCl | aqueous | 水溶液 |
| Sodium hydroxide | 氢氧化钠 | NaOH | solid | 固体 |
| Sodium chloride | 氯化钠 | NaCl | solid | 固体 |
| Magnesium oxide | 氧化镁 | MgO | solid | 固体 |
| Iron(III) oxide | 氧化铁(III) | Fe₂O₃
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